1. Field of the Invention
The present invention relates to a high strength steel sheet useful as automobile, building, electric or other members and a process for producing the same. More particularly, the present invention relates to a high strength hot-dip galvanized steel sheet which has improved bulging formability at the time of press forming and improved plating adhesion and a process for producing the same. The term “high strength hot-dip galvanized or galvannealed steel sheet” as used herein includes high strength hot-dip galvanized steel sheets (GI) and high strength hot-dip galvannealed steel sheets (GA).
2. Background Art
To meet a demand for lower fuel consumption of automobiles and the like in recent years, studies have been made on a weight reduction of members such as cross members and side members of automobiles and the like. Regarding materials, studies on an increase in strength have been made from the viewpoint of ensuring strength even in reduced thickness of members.
In general, however, increasing the strength of materials results in deteriorated press formability of the materials. Therefore, in order to realize the weight reduction of the above members, steel sheets capable of simultaneously satisfying good press formability and high strength are required, and the development of such steel sheets has been desired in the art.
Among index values for formability are included elongation and, in addition, n value and r value in tensile tests. At ask to be accomplished in recent years is to simplify the step of pressing through one-piece molding. Therefore, what is particularly important is that the n value corresponding to uniform elongation is large.
To this end, hot rolled steel sheets or cold rolled steel sheets, which utilize transformation induced plasticity of retained austenite present in steel, have been developed. In these steel sheets, only about 0.07 to 0.4% of carbon (C), about 0.3 to 2.0% of silicon (Si), and about 0.2 to 2.5% of manganese (Mn) are used as basic alloying elements without incorporating any expensive alloying element, and heat treatment characterized by, after annealing in a two-phase region, performing bainite transformation at a temperature of about 300 to 450° C. is carried out to allow retained austenite to exist in the metallurgical structure. These steel sheets are disclosed, for example, in Japanese Patent Laid-Open Nos. 230715/1989 and 217425/1990.
Steel sheets, to which the above technique can be applied, are not limited to cold rolled steel sheets produced by continuous annealing. For example, Japanese Patent Laid-Open No. 79345/1989 discloses that this technique can also be applied to hot rolled steel sheets by controlling cooling temperature of a coiling or a runout table.
The production of automobiles of higher grade has led to a tendency toward coating of automobile members with a view to improving corrosion resistance and appearance. At the present time, galvanized steel sheets are used in most of the automobile members except for specific members mounted within the automobiles. Accordingly, from the viewpoint of corrosion resistance, coating which is useful for these steel sheets is hot-dip galvanizing, or hot-dip galvannealing wherein, after hot-dip galvanizing, alloying treatment is carried out. Among these high-tensile steel sheets, steel sheets having a high silicon content are likely to form an oxide film on the surface thereof. This poses problems including that very small non-coated sites occur in hot-dip galvanizing, or plating adhesion of the worked part after the completion of alloying is deteriorated. Thus, at the present time, any high-silicon-base high-tensile and high-ductile hot-dip galvannealed steel sheets possessing excellent plating adhesion of the worked part and, at the same time, excellent corrosion resistance have not been put into practical use.
In the steel sheets disclosed, for example, in Japanese Patent Laid-Open Nos. 230715/1989 and 217425/1990, however, it should be noted that 0.3 to 2.0% of silicon is contained and the retained austenite is ensured by utilizing unique bainite transformation of the steel. Therefore, the contemplated metallurgical structure cannot be provided without considerably close control of cooling after annealing in an intercritical temperature range or holding in a temperature range of about 300 to 450° C., and strength and elongation disadvantageously deviate from the target range.
This heat history can be realized on a commercial scale by continuous annealing equipment or a runout table after hot rolling and the step of coiling. Since, however, the transformation of austenite is rapidly completed at 450 to 600° C., control should be carried out so that the residence time is short particularly at 450 to 600° C. Further, even at 350 to 450° C., the metallurgical structure undergoes a significant change depending upon the holding time. Therefore, when the heat treatment conditions have deviated from predetermined conditions, only unsatisfactory strength and elongation are provided.
Further, since the residence time at 450 to 600° C. is long and a large amount of silicon, which deteriorates suitability for coating, is contained as an alloying element, the production of a coated steel sheet by passing the steel sheet through hot-dip galvanizing equipment is impossible. After all, poor surface corrosion resistance of the steel sheet containing about 0.3 to 2.0% of silicon disadvantageously hinders the widespread commercial use of the steel sheets.
In order to solve the above problem, for example, Japanese Patent Laid-Open Nos. 247586/1993 and 145788/1994 disclose steel sheets having suitability for coating improved by regulating the content of silicon. In the technique disclosed in the above publications, retained austenite is formed by adding aluminum (Al) instead of silicon. However, as with silicon, aluminum is more likely to be oxidized than iron (Fe). Specifically, aluminum is likely to form an oxide film on the surface of the steel sheet. Therefore, disadvantageously, satisfactory plating adhesion cannot be ensured.
Further, for example, Japanese Patent Laid-Open Nos. 333552/1992 and 346644/1992 disclose a hot-dip galvannealing method for a high-silicon-base high-strength steel sheet. In this method, after pre-coating of nickel (Ni), the pre-coated steel sheet is rapidly heated at a low temperature and is subjected to hot-dip galvanizing, followed by alloying treatment. Since, however, this method requires nickel pre-coating, disadvantageously, equipment for nickel pre-coating should be newly provided.